Reference signals are used in most wireless communication systems for various purposes such as:                Obtaining time and frequency synchronization;        Enabling a user equipment (UE) to measure channel quality and demodulate data; and/or        Discovering network or transmission points        
In the 3rd-Generation Partnership Project (3GPP) so-called Long-Term Evolution (LTE) systems, almost all reference signals are sent periodically. The UE knows the periodicity and the subframe number offset for the reference signals either by specification or by configuration. For example, the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are sent with a periodicity of 5 ms and with a subframe offset of 0, i.e., PSS/SSS are sent in subframes (SF) whose subframe number fulfills the condition:SF(mod 5)=0.
Another example is Channel-State Information Reference Signals (CSI-RS) measurement resources, where a UE is configured to estimate channel properties based on a specific CSI-RS transmitted at specific time instances, which are determined by a subframe period (SF_period) and a subframe offset (SF_offset). The standard supports various values for SF_period and SF_offset. For example, SF_offset=4 and SF_period=20 is one possible configuration which means that a UE should estimate the channel in subframes that fulfill the condition:SFN*10+SF(mod SF_period)=SF_offset,where SFN is the system frame number (SFN is incremented each time SF wraps, i.e., every time SF goes from 9 to 0).
License-Assisted Access via LTE (LAA-LTE) has recently been proposed as a technology for coexistence on unlicensed carriers with, e.g., WiFi. On an unlicensed carrier all data transmissions are to be controlled by a Listen-Before-Talk (LBT) mechanism, while control and management signaling can be sent without LBT at a maximum duty cycle of 5% within an observation period of 50 ms. The LBT is a functionality which means that the channel is assessed to be clear before transmission begins. Since CSI-RS can be considered a type of control/management signaling, LAA-LTE may send a discovery signal (a.k.a. beacon signal) including required reference signals, such as PSS, SS, and/or extended Synchronization Signal (eSS) and possibly also CSI-RS without LET, and may hence preserve the periodicity of the reference signals.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.